I think people were thinking that since this is kind of a special occurrence and not your run of the mill engagement Kate might make an appearance with what looks like a good number of her in-laws - particularly since they made an announcement about how she'll be at Wimbledon.
It would be unfair of us if William was going to an event for one of his charities and people were saying how they hope Kate goes too or how they're disappointed she isn't going. But this isn't just one of his charities - this is kind of a big thing that stands out a bit. It seems to be a bit more in line (to me) with the Trooping or Remembrance Day than your typical engagement so it's reasonable to wonder if Kate might attend it despite her maternity leave - she did, after all, attend the Trooping, and doing the occasional big family engagement like this one isn't going to prevent her from being a mother.
Women on mat leave may get a chunk of time off (how much varying depending on who and where they are), but that doesn't mean they just spend the whole period bonding with the baby. Kate doing a sporadic engagement here or there because it's a big deal wouldn't take away from her maternity leave, and people wondering if she might and being disappointed if she doesn't isn't actually a criticism of her.